Disentangling Effects of Nuclear Structure in Heavy Element Formation

Abstract

Forming the same heavy compound nucleus with different isotopes of the projectile and target elements allows nuclear structure effects in the entrance channel (resulting in static deformation) and in the dinuclear system to be disentangled. Using three isotopes of Ti and W, forming 232Cm, with measurement spanning the capture barrier energies, alignment of the heavy prolate deformed nucleus is shown to be the main reason for the broadening of the mass distribution of the quasifission fragments as the beam energy is reduced. The complex, consistently evolving mass-angle correlations that are observed carry more information than the integrated mass or angular distributions, and should severely test models of quasifission.

Shape evolution and collective dynamics of quasifission in the time-dependent Hartree-Fock approach

“…In this case, each orientation of the deformed nucleus may encounter a different mean-field evolution [82,83]. Such orientation dependence of reaction mechanisms has been experimentally studied in quasifission with actinide targets [84][85][86][87][88][89][90] and confirmed in TDHF studies [25,33,42]. In particular, these studies have shown that collisions of a spherical projectile with the tip of prolately deformed actinides lead to fast quasifission (with contact time smaller than 10 zs) while collisions with the side of the actinide may induce longer contact times, larger mass transfer, and possible fusion.…”

Systematic study of quasifission characteristics and timescales in heavy element formation reactions

“…To define the smooth trends in quasifission, a large number of MAD measurements have been selected, for beam energies somewhat above the capture barrier (typically by ∼ 6%). Here the known effects of deformation alignment [10,11,17,19] and shell structure observed in measurements at below-barrier energies [14,20] are much reduced [11,21,22]. However the beam energies should not be too far above the capture barriers, otherwise high angular momenta would be introduced in the collisions, which would then not be representative of heavy element formation reactions.…”

“…The story is very different at sub-barrier energies. Here it is wellknown that collisions with the tips of deformed nuclei are those that lead to capture [4], and that fission angular distributions [17], mass distributions [10,13,25] and MAD [11,16] point to the changing nature (shorter sticking time) of quasifission under these circumstances. Microscopic TDHF calculations of quasifission masses and angles give a good match [16] to experimental results in reaction 40 Ca+ 238 U, across the transition from sub-barrier to above-barrier energies (at which all collision orientations contribute).…”

“…Quasifission mass-angle distributions (MAD) first measured at GSI in the 1980s [2,5] showed that quasifission timescales could often be shorter than the rotation time of ∼10 −20 s. However, subsequently only a few measurements [6,7] were made until recent years, when an extensive series of experiments (using the Australian National University Heavy Ion Accelerator Facility and CUBE spectrometer) were carried out [3,[8][9][10][11][12][13][14][15][16]. The kinematic coincidence technique used in the measurements [2,3,17] provides direct information on the mass-ratio of the fragments at scission; thus, the data are represented in terms of mass ratio M R , rather than pre-or According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3].…”

“…There is a clear correlation between the MAD class and the entrance channel charge product. Other entrance channel characteristics are important in determining the sticking times and MAD characteristics, including neutron richness [14,18], and shell structure including static deformation [11] and magic numbers [14].To improve our quantitative understanding of the role of shell structure in the dynamics of quasifission, we make an analogy with the liquid drop model approach to nuclear masses, in which localized shell effects can be quantified when the underlying smooth (liquid drop) trends are well defined. To define the smooth trends in quasifission, a large number of MAD measurements have been selected, for beam energies somewhat above the capture barrier (typically by ∼ 6%).…”